Studies On The Effects Of Mixed Eucalyptus Grandis Plantations On Aboveground And Belowground Biodiversity

Eucalyptus spp.managed typically with large-scale monoculture plantations make great contributions to economic development.Meanwhile,Eucalyptus plantations have resulted in many severe ecological problems including reduced biodiversity,degraded soil,and decreased productivity.Compared to monoculture plantations,mixed plantations generally have multispecies composition,crown complementary,and higher environmental heterogeneity,which could present niche separation,then reduce competition among tree species and improve effectively resources use,generally generating positive mixing effects on ecological functions and productivity.The predominant driver for mixing effects in mixed plantations is the interactions among component tree species.Compared to aboveinteractions,below-interactions among the species mainly regulated by roots are more complex and define productivity and sustainable development in mixed plantations to a large extent.Recently,studies on mixed plantation pay more attentions to facilitation on growth and productivity,belowground interactions among tree species in mixed plantations remain unclear.Based on previous studies on the effects of stand ages of Eucalyptus grandis on biodiversity and allelopathy,four mixed E.grandis plantation(E+B E.grandis + B.luminifera,E+A E.grandis + A.formosana,E+C E.grandis + C.lanceolata,E+P E.grandis+ P.massoniana)were selected in western Sichuan province,southwestern China.Monoculture E.grandis plantations were selected as controls.We investigated understory plant diversity,soil fauna biodiversity,and bacteria diversity in rhizosphere soils of E.grandis in different E.grandis plantations,identified root exudates and analyzed chemical defense variation,examined morphological and physiological traits of roots of E.grandis,tested bacterial community compositions by high-throughput sequencing,soil physicochemical characteristics and extracellular enzyme activities in rhizosphere soils of E.grandis.The study aimed to examine the effects on above-and below-ground biodiversity in E.grandis plantations when mixed with different tree species and its mechanisms of belowground among tree species,which could provide theorical supports for the selection of neighbors when constructing mixed E.grandis plantations.The main results showed that:(1)Compared to pure 4-year-old E.grandis plantations,the diversity of plants,soil fauna,and rhizosphere soil bacteria increased in all mixed E.grandis plantations.The biodiversity in E+B and E+A were higher than that in pure plantations and the other mixed plantations.Regardless of woody and herbaceous plants,their species number,density,and Shannon index in mixed E.grandis plantations were the highest in E+A plantations.The number of groups and density of soil macrofauna in each sampling time,the Shannon index of all fauna and the group numbers of soil meso-and micro-arthropods in 4 and 7 months and their density in 7 months were all higher in E+A plantations.The OTU number,diversity and richness index of bacteria in rhizosphere soil of E.grandis were higher in E+A and E+B plantations,and generally reaching the highest in E+A plantations.(2)Compared to pure E.grandis plantations,the morphological characteristics and distribution patterns of fine roots of E.grandis changed significantly when mixing with different tree species.In the shallower soil layer(0-40 cm),specific surface area and root biomass density were highest in E+P and E+B mixed plantations,respectively.In the deeper soil layer(40-80 cm),root biomass density and root surface area density and root length density in 60-80 cm were significantly increased in E+A stands as compared to pure and other mixed plantations.Root respiration rate of E.grandis were the highest in E+A stands.The activities of malic dehydrogenase were higher in E+B stands and glucose-6-phosphate dehydrogenase activities were higher in E+A and E+C plantations.(3)Compared to pure E.grandis plantations,the amounts of potential allelochemicals in differential metabolites were lower in mixed E.grandis plantations.Higher abundant compounds in potential allelochemicals including phenolic acids,aldehydes,eicosanoids,and glycosides were less abundant in mixed plantations than in pure plantations,and their abundant were the highest in 4E.Phenolic acids were less abundant in E+B,E+A,and E+C plantations.The amounts of aldehydes were the lowest in E+A plantations.Compared to other stands,eicosanoids were less abundant in E+B and E+A plantations.However,alkaloids were more abundant in mixed plantations than in pure plantations.(4)The bacterial community compositions in rhizosphere soils of E.grandis were altered significantly in pure and different mixed plantations.The dominant bacterial phyla in each plantation were protobacteria,acidobacteria,chloroflexi,actinobacteria,and bacteroidete.Compared to pure stands,dominant bacteria at family level were more abundant in mixed plantations.especially in E+B and E+A plantations.Additionally,when mixed with alder,rhizosphere soils in E.grandis recruited more lignin and cellulose degrading and phosphate solubilizing beneficial bacteria,such as families Lachnospiraceae,Steroidobacteraceae Ruminococcaceae and Xanthomonadaceae.(5)The activities of most of enzymes related to C,N,and P cycle in each sampling time were higher in E+P plantations,but lower in E+A plantations.The activities of peroxidase polyphenol oxidase in each sampling time were all the highest in E+P mixed plantations.For all plantations,the values of rhizosphere soil enzyme C:N ratios were more than 1,while enzyme C:P and N:P ratios values were less than 1.The vector lengths of all plantations were more than 1,and vector angles were greater than 45°.In mixed plantations,vector angle decreased significantly in E+A mixed plantations.In each sampling time,the p H,soil organic matter,total nitrogen,and available phosphorus in rhizosphere soils of E.grandis were greatly higher in E+A mixed plantations than in other plantations.In conclusion,compared to 4-year-old E.grandis plantations,positive mixing effects on above-and below-ground biodiversity were widely observed in all E.grandis mixed plantations.In mixed plantations,mixing E.grandis with alder exerted optimal ecological mixing effects on biodiversity.When mixed with alder,E.grandis increased fine roots production in deeper soil layer,reduced the release of defense chemicals,recruited more beneficial bacteria,and alleviated P restriction of rhizosphere soil microorganisms,as well as improved rhizosphere soil nutrient.These ecological processes caused niche separation,reduced competition between E.grandis and alder,improved resource utilization.Therefore,facilitation belowground between E.grandis and alder were presented,which furthermore mediated positive mixing effects on aboveground and belowground biodiversity.The results could provide theoretical supports for reasonable selection of neighbors in constructing mixed E.grandis plantations.